DHCP not working after connecting second Service Provider Link

[Hythim Ali El Hadad]

Hi

Our branches are connected through WAN using 2 Links

One main from one service provider and secondry through another service provider and 3rd link 3G

1st and 2nd ISP links configured with BGP, all traffic go through 1st link as primary. Secondry link send traffic only when 1st link go down

The problem is that some sites LAN stop working as no DHCP inside the LAN when connecting the secondry link

When removing the 2nd link or shut down the interface, the LAN is working again

What could be the issue, I remember that I see the same issue earlier with another setup and found the problem at service provider site, they are advertising my subnets as /29 instead of /28 as I remeber

What could be the problem here ?

Thanks

[balaji.bandi]

The problem is that some sites LAN stop working as no DHCP inside the LAN when connecting the secondry link

I do not believe this is related to BGP config - but we are not sure how is your setup configured, how are you reaching DHCP?

is this BGP on the internet? what Router is this ? Would you happen to have any configuration and topology for us to understand the issue ?

[MHM Cisco World]

if you config DHCP relay and the second link not provide the return back path from DHCP server to DHCP relay (router or SW) then the LAN client failed to get IP from DHCP. 
connect second link and do ping from DHCP relay (Router or SW) to DHCP server.

[Hythim Ali El Hadad]

Thanks for your replies

The DHCP server is internal to the site inside the site

But as you see the DHCP work again after disabling the secondry ISP link

[balaji.bandi]

The DHCP server is internal to the site inside the site

if this is inside the site on the same device?  (I think you need to provide the config as requested before - so it's hard to say what is wrong and what is configured.)

[MHM Cisco World]

DHCP server internal to site are it config with DHCP relay ??

[paul driver]

Hello
Can you provide further information and if possible a topology of your branch sites please, You don’t mention if you are sung single or dual wan rtrs, what dynamic ibgp routing ( if any) is running on the lan, what device is servicing the inter-vlan routing and how the lan is connected to your wan rtrs